Joints between structural members are often formed by overlapping the two members together to create a lap joint. The lap joint is secured together by various means. In some instances, holes are drilled and rivets or other fasteners are disposed through the holes. Such overlapping surfaces can also be joined by welding the structural members together using one or more welds at the overlap of the materials. One type of welding process used to join lap joints is friction stir welding (FSW) in which a rotatable pin extends in a direction generally perpendicular to the interface of the members and is urged through the members along the interface.
However, the surfaces of each piece of joined metal adjacent the fasteners or welds mated by the lap joint, called “fay” surfaces, are often not fully bonded by the weld or other fasteners. The resulting spaces in the interface are open to moisture, chemicals, debris, and other foreign materials. This can result in increased corrosion of the structural members. In FSW, for example, the resulting nugget is typically not as wide as the interface of the overlapping members, such that the members define spaces in the interface in which corrosion can occur.
In order to control corrosion in these joints, sealants are often disposed in the spaces prior to, during, and/or after welding. Conventional sealants are known to reduce moisture which may be trapped between the faying surfaces and/or brought in by capillary action. These sealants are also useful for reducing mechanical and fatigue problems resulting from rubbing between the faying surfaces, vibration, and the like. Conventional faying surface sealants include polysulfide, polythioether, and the like, which are applied to the mating surfaces prior to joining.
However, many conventional sealants are degraded by the heat and mechanical activities involved in the welding process, as well as by vibration during use of the joined component. In some instances, the sealants become loosened from the interface. Specifically, when the sealant is applied before welding, care must be taken to avoid introducing the sealant into the nugget of the joint because the sealant can negatively impact the strength and/or corrosion resistance of the joint. This can involve placing masking tape on the area of the structural members where the nugget will be formed, disposing the sealant on the structural members, and then removing the masking tape to leave a relatively clean area for forming the nugget. However, such a process is time consuming. Additionally, even if such precautions are taken, some of the sealant can be squeezed into the interface as members are positioned and urged together to form the joint. The sealant then mixes with the plasticized material of the joint, thereby reducing the quality of the weld joint.
Therefore, what is needed are improved sealants and methods for forming a corrosion resistant joint in structural members.